Write the ending chunk and, in chunked encoding, a trailer to the
wire.

Write the ending chunk and, in chunked encoding, a trailer to the
wire. If a request is cancelled, or the stream is closed this
method should return a failed Future with Cancelled as the
exception, or a Future with a Boolean to indicate whether the
connection is to be closed or not.

chunk

BodyChunk to write to wire

returns

a future letting you know when its safe to continue (if false) or
to close the connection (if true)